1. Field of the Invention
The present invention relates to a projection apparatus for transferring a pattern formed on a mask by projecting it onto a substrate with a charged particle beam having an arcuate cross-section, and a method of controlling the same.
2. Description of the Related Art
Conventionally, in mass production of semiconductor memory devices, an optical stepper having high productivity has been used. In the production of new-generation memory devices from 1G- and 4G-DRAM memory devices having a line width of 0.2 μm or less, the high-productivity charged particle beam exposure method having high resolving power and using charged particles, e.g., electrons or ions, is a promising technique that replaces the optical exposure method.
The electron beam exposure method as an example of the charged particle beam exposure method is mainly performed in the forms of a Gaussian beam method and a variable shaped beam method, and accordingly has a low productivity. Therefore, the electron beam exposure method has been used in only applications such as mask making, research and development of VLSIs, and the manufacturing process of ASIC devices on small-lot production, where the characteristics of the excellent resolution performance of the electron beam are effectively used. To employ the electron beam exposure method in mass production, an increase in its productivity is a significant problem.
In recent years, a cell projection method has been proposed as one method that solves this problem. According to this method, the repeated portion of the memory circuit pattern is divided into cells each having several μm regions, and the pattern is exposed in units of cells. With this method, the maximum region that can be exposed at once has a size as small as about several μm. A plurality of deflectors are used to enlarge the exposure region. As the exposure region becomes large, deflection aberrations increase. These deflection aberrations are eliminated by dynamic correction using focus coils and stigmators. This method can enlarge the maximum region that can be exposed at once. However, it takes a comparatively long period of time till the deflected electron beam is settled at a desired position. This decreases the productivity.
An electron beam projection exposure method is under development which does not require a time for settling the electron beam. The projection system of an apparatus of this type uses a symmetric magnetic doublet lens. Also, an aperture for separating scattered electrons and unscattered electrons at the pattern portion and mask membrane portion, respectively, of the transfer mask from each other is arranged at a position that divides the distance between the mask and a photosensitive member in accordance with the magnification ratio. The positions of the principal planes of the two magnetic lenses of the magnetic lens are respectively set between the mask and the aperture, and at the intermediate position of the distance between the aperture and a sample coated with the photosensitive member. The two magnetic lenses can move only a small distance when they are mechanically adjusted. According to the electron beam projection exposure method, the pattern to be transferred onto the sample is divided into a plurality of partial patterns, and the divided partial patterns are formed on a mask. While an electron beam irradiates a selected partial pattern on the mask, the mask and sample are continuously moved in opposite directions. An electron beam transmitted through the mask irradiates the sample, thereby exposing the sample. Since this method does not perform electron beam scanning, it does not take much time to settle the electron beam. As a result, this method has a higher productivity than other methods described above.
In the case of the electron beam projection apparatus described above, the exposure width of one exposure operation is generally several mm to several ten mm. Image distortion and off-axis aberration mainly caused by field curvature of the projection lens become larger than in the conventional apparatus, thus degrading the exposure pattern's resolution. In order to prevent degradation in the resolution, the region of the pattern of the mask to be irradiated with the electron beam may be reduced. However, this cannot achieve a great improvement in productivity as compared to the conventional electron beam exposure method.